Sifting method



R. BACH SIFTING METHOD Dec. 15, 1964 Filed June 28, 1960 Inventor.

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United States Patent 3,161,587 srrrnuc Martian Robert Bach, Basel, Switzerland, assignor to Lonza Elec- The present invention relates to a method of sifting, and more particularly to the sifting of pulverulent materials such as synthetic material which, due to the fine ness of the individual particles and due to their ability to accept electrostatic charges, up to now could be sifted only with great difficulties. The smaller the size of the individual particles, the greater will be the difliculties in sifting the same, and the greater will be the unfavorable influence of electrostatic charges on the sifting process. This might go so far that agglomerations of the pu verulent material are formed which defy further sifting attempts.

Various remedial measures have been proposed in order to overcome the above described diiliculties. For instance, it has been suggested to add to the pulverulen material so-called antistatic substances. However, these antistatic substances tend to find their way into the end product and might interfere with the intended use of the product. Ionization of the sifting chamber also does not give satisfactory results in many cases and furthermore requires relatively complicated and expensive installations, the operation of which involves certain dangers. Attempts to operate in an atmosphere of relatively high humidity, for instance, 70 to 80% relative humidity, such as are used with some success in the textile industry, particularly, in spinning and weaving operations, were not effective for improving sitting of finely particulate material. It has also been suggested to subject a mixture of the finely particulate material and of Dry Ice to sifting, for instance, in a moist nitrogen atmosphere, thereby condensation of moisture is accomplished and thus the ele trostatic charging is to be counteracted. However, it was found that particularly in large scale operations, it was not possible in this manner to achieve an even cooling of the particulate material, and furthermore, condensation took place primarily in the immediate vicinity of the relatively large Dry Ice particles, causing caking of the pulverulent material as well as ice formation which tended to clog up the meshes of the sifting device.

It is therefore an object of the present invention to overcome the aforementioned difiiculties in the sifting of finely pulverulent materials.

It is a furt er object of the present invention to provide a method for the sifting of finely particulate materials which are capable of accepting electrostatic charges, which method can be carried out in a simple and economical manner.

it is still another object of the present invention to provide a method for the sifting of electrostatic chargeable pulverulent material such as synthetic thermoplastic materials which method can be carried out on a large scale and will give a high yield of sifted material.

Other objects and advantages of the present invention will become apparent from a further reading of the de- 2 justed that upon contact between the pulverulent material and the atmosphere condensation will take place at the surface of the particles of the pulverulent material forming a thin film of water thereon capable of eliminating electrostatic charges of the particles, and sifting the pulverulent material While being exposed to the atmosphere and thus substantially free of electrostatic charges.

According to a preferred embodiment, the present invention includes a method of sifting electrostatically chargeable pulverulent, thermoplastic, polymerized material, comprising the steps of introducing the pulverulent material having a first predetermined temperature into an atmosphere having a second predetermined temperature being higher than the first predetermined temperature and having a relative humidity of at least about the first and second predetermined temperatures being between 5 C. and 50 C. and the temperature difference between the second and the first predetermined temperatures being between 0.5 C. and 5 C. and sufficiently high to cause water condensation on a body having the first predetermined temperature, so that moisture condensation will take place only at the surface of the particles of the pulverulent material forming a thin film of moisture thereon when the material contacts the atmosphere, thereby making the particles substantially incapable of carrying electrostatic charges, and sifting the particulate material while the same is thus substantially free of electrostatic charge-s.

Thus, according to the present invention, the sifting of electrostatically chargeable pulverulent materials, particularly powders of synthetic materials, is carried out in moist atmosphere preferably having a temperature of between 5 and 50 C. and a relative humidity of between 95 and whereby the particulate material which is to be sifted will be brought in contact with the sieve ata temperature which is below the temperature of the surrounding moist atmosphere so that moisture condensation will take place at the surface of the particles which are to be sifted, forming a very thin film of moisture such as water thereon which film will effectively counteract an electrostatic charging of the particles. In order to effectively operate the method of the present invention, it is essential that condensation takes place only at the surface of the particles which are to be sifted; however, the dew point of the remainder of the surrounding atmosphere will not be exceeded. in other words, water droplet formation which mi ht lead to lumping of the pulverulent material must be avoided.

It has been found that these conditions can be relatively easily met by, on the onerhand, providing an atmosphere of high relative humidity, preferably at least 95% and up to the dew point, and, on the other hand, introducing the particulate material into the atmosphere at a somewhat lower temperature which is sufiiciently lower than the temperature of the atmosphere to cause moisture condensation on the surface of the particles, but not so low as to reduce the temperature of the entire atmosphere to such an extent that the relative humidity thereof would rise above the dew point.

Preferably, the relative humidity of the atmosphere is adjusted at or as closely as possible below the dew point at the working temperature. And, as stated above, care must be taken that the temperature differential between the atmosphere of high relative humidity and the particulate material which is to be sifted is not too great in order to avoid harmful general condensation with mist or droplet formation in the atmosphere within the sifting device. Thetemperature difference between the atmosphere and the material to be sifted should be so adjusted that uponbontact between the material and the atmosphere the amount of water-which is precipitated dividual particles of the pulverulent material, a liquid layer of molecular magnitude.

Preferably, the temperature of the pulverulent material will be adjusted to between 0.5 and C. below the temperature of the atmosphere in the sifting device. For instance, pulverulent material maintained at a temperature of C. may be effectively sifted in an atmosphere of 100% relative humidity having a temperature of 13 C.

It has been found advantageous to maintain the place in which the sifting device is located at an even temperature preferably equal to the temperature of the material which is to be sifted. Thus, if for example a pulverulent material having a temperature of 10 C. is to be sifted in a sifting device in which an atmosphere of high relative humidity and having a temperature of 13 C. is maintained, then the room in which the sifting device is located will be preferably maintained at a temperature of 10 C., by suitably insulating and venting the same. In such room or enclosure maintained at a constant temperature, a closed moist air circuit is arranged which will supply air of the desired temperature and high relative humidity to the sifting device. This may be accomplished in open or closed systems. For instance, in an open system, fresh air may be sucked through a Raschig tower in which the temperature of the air is adjusted and the same is substantially saturated with water. In closed systems, the air after passing through the sieve will be sucked in by a fan, filtered and then passed through the tower and back into the sifting device. Any of the conventional sieve arrangements for the sifting of particulate material may be used in accordance with the present invention.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following descniption of specific embodiments when read in connection with the accompanying drawings, in which the drawing is a schematic elevational view of a device for carrying out the method of the present invention.

Referring now to the drawing, a thermo-insulated enclosure 1 is shown in which the air moistening device 2 and the sifting and storage device 3 are arranged. The interior of enclosure 1 is maintained at the temperature of the material which is to be sifted.

The air moistening device 2 comprises moistener 4, water pump 5, Water heater 6, condensate separator 7, water introducing conduit 3 and water conduits 9, 9", 9' and 9", as well as air introducing conduit 10, blower 11 and moist air conduits 12' and 12". The moist air is passed through conduit 12" and flexible conduit 19 into sifting device 15.

The storage and sifting device 3 comprises the hopper 13 for storing pulverulent material to be sifted, the dosing device 14 and the vibrating sifting device 15 with the screen 21. Flexible conduits connect sifting device 15 with storage hoppers 16 and 17 for the sifted material and for the coarse overflow. Conduit 18 serves for outward passage of moist atmosphere from sifter 15. The moist atmosphere removed through conduit 18 is then passed through a filter device (not shown) and may then be returned through air conduit 10. The entire device may be automatically controlled by means well known per se.

The method according to the present invention is suitable for the sifting of a great variety of pulverulent materials, provided that the material will neither be dissolved nor caused to swell when in contact with water or moist air. Excellent results were obtained by sifting in accordance with the present invention electrostatically chargeable powders such as thermoplastic polymerization products for instance the polymerizates and acrylate is introduced in the illustrated apparatus.

mixed polymerizates of vinyl chloride and vinyl acetate, styrene, acrylonitrile, methyl methacrylate and the like. The range of particle sizes which may be sifted according to the present invention is determined by the mesh widths of the sieve installation. Preferably, the particle diameter of powders which are sifted according to the present invention will be Within the magnitude of from about 2 mm. to about 2 microns.

The following examples are given as illustrative only of the present invention, however without limiting the invention to the specific details of the examples.

Example I In a device such illustrated in the drawing, pulverulent polyvinyl chloride free of any additional substances and in the state in which it is received from the polymerization process and having a temperature of 20 C. is introduced at a rate of 600 kg./h. The polyvinyl chloride flows evenly onto a screen, moving in a horizontal plane in circular translation, of 0.6 m. screening area, having mesh openings of 0.2 mm. Simultaneously, air which has been saturated with water at a temperature of 22 C. is blown through the sifter 15. Thus, the temperature differential between the pulverulent polyvinyl chloride and the surrounding atmosphere upon contact in sifter 15 will be 2 C. In the sifter, 10% of the charge is separated as coarse material and W of the separated coarse material, in fact, are particles having a size greater than 0.2 mm. The increase in moisture content of the sifted portion consisting of particles having a size not exceeding 0.2 mm. and amounting to of the charge is only 0.1% by Weight and thus practically negligible.

By sifting identical material under the conditions described above, however, without the introduction of moist air, it is found that after operating the device for about one half hour, 90% of the charge are retained as coarse material and inspection of the sieve shows that most of the mesh openings are plugged up. Furthermore, particularly, the finer particles tend more and more to form agglomerations.

Example II Pulverulent freshly polymerized polyvinyl chloride is introduced into the illustrated apparatus at a rate of 600 kg./h. The temperature of the pulverulent material is maintained at about 10 and 11 C., and the temperature of the moisture saturated air at between 13 and 14 C. The moist air is passed through the sitting device at a rate of 20 mfi/h.

It is found that 11% are retained as coarse material and that only of these 11% consists of particles having a diameter of less than 0.2 mm.

Example III Pulverulent polyvinyl chloride having a temperature of 35 C. is introduced into the illustrated device in the manner described in Example I, and at a rate of 600 kg./h. Water saturated air of 36 C. is blown through the sifting device at the rate of 20 m. /h. After one hour, it is found that 540 kg. of the material have passed through the sieve and that of the 60 kg. retained as coarse particles, 53 kg. in fact consist of particles having a diameter greater than 0.2 mm.

Example IV At a rate of 600 kg./h. pulverulent polymethyl meth- The temperature of the pulverulent material is maintained at 10 C. and the temperature of the moist air at 14 C. The moist air of 96% relative humidity is passed through the sifting device at a rate of 20 m. /h.

Only about of the retained coarse material which latter amounted to 14% of the total amount of substance to be sieved consisted of particles having a diameter of less than 0.2 mm.

To'establish the best temperature differential between the electrostatically chargeable pulverulent material and the moist atmosphere having a predetermined temperature, within the limits as set forth herein above, it is preferred to start the introduction of the material to be sieved into the sifter while the moist air stream has the same temperature as the said material, and then gradually increasing the temperature of the moist air stream until the optimum throughput is secured. In general the beginning of the actual sifting is very sudden, when the correct temperature range is attained.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be secured by Letters Patent is:

l. A continuous process of sifting electrostatically chargeable, pulverulent material, comprising the steps of introducing pulverulent material having a first predetermined temperature of introduction into a closed chamber; introducing into said closed chamber also a gaseous medium having a second predetermined temperature of introduction which is higher than said first predetermined temperature of introduction and the temperature difference between said first and second predetermined temperatures being between about 0.5 C. and 5 C., said gaseous medium having a relative humidity of between 95% and 100% sufficiently high to cause moisture condensation on a body having said first predetermined temperature; said pulverulent material having said first predetermined temperature of introduction mixing in said closed chamber with said gaseous medium having said second predetermined temperature of introduction and having said sufiiciently high relative humidity so that said gaseous medium is cooled by said pulverulent material and so that moisture condensation will take place on the surface of the particles of said pulverulent material forming a thin film of moisture thereon thereby making said particles substantially incapable of carrying electrostatic charges; and sitting said pulverulent material in said closed chamber while the same is thus substantially free of electrostatic charges, whereby a sifted material is obtained substantially without increasing the moisture content thereof.

2. A continuous process of sifting electrostatically chargeable, pulverulent material, comprising the steps of introducing a pulverulent material selected from the group consisting of polymerized vinyl chloride, vinyl acetate, styrene, acrylo nitrile and mixed polymerizates thereof and having particle sizes substantially within the range of between 2 mm. and 2 microns having a first predetermined temperature of introduction into a closed chamber; introducing into that closed chamber also a gaseous medium having a second predetermined temperature of introduction which is higher than said first predetermined temperature of introduction, said first and second predetermined temperatures being between 5 C. and C. and the temperature difference between said first and second predetermined temperatures being between 0.5 C. and 5 C., said gaseous medium having a relatively high relative humidity of at least and sufficiently high to cause moisture condensation on a body having said first predetermined temperature of introduction; said pulverulent material having said first predetermined temperature of introduction mixing in said closed chamber with said gaseous medium having said second predetermined temperature of introduction and having said relatively high humidity so that said gaseous medium is cooled by said pulverulent material and so that moisture condensation will take place on the surface of the particles of said pulverulent material forming a thin film of moisture thereon thereby making said particles substantially incapable of carrying electrostatic charges; and sitting said pulverulent material in said closed chamber while the same is thus substantially free of electrostatic charges, whereby a sifted material is obtained substantially without increasing the moisture content thereof.

References Cited in the file of this patent FOREIGN PATENTS Germany Aug. 1, 1957 Canada Mar. 13, 1956 OTHER REFERENCES 

1. A CONTINUOUS PROCESS OF SIFTING ELECTROSTATICALLY CHARGEABLE, PULVERULENT MATERIAL, COMPRISING THE STEPS OF INTRODUCING PULVERULENT MATERIAL HAVING A FIRST PREDETERMINED TEMPERATURE OF INTRODUCTION INTO A CLOSED CHAMBER; INTRODUCING INTO SAID CLOSED CHAMBER ALSO A GASEOUS MEDIUM HAVING A SECOND PREDETERMINED TEMPERATURE OF INTRODUCTION WHICH IS HIGHER THAN SAID FIRST PREDETERMINED TEMPERATURE OF INTRODUCTION AND THE TEMPERATURE DIFFERENCE BETWEEN SAID FIRST AND SECOND PREDETERMINED TEMPERATURES BEING BETWEEN ABOUT 0.5*C. AND 5*C., SAID GASEOUS MEDIUM HAVNG A RELATIVE HUMIDITY OF BETWEEN 95% AND 100% SUFFICIENTLY HIGH TO CAUSE MOISTURE CONDENSATION ON A BODY HAVIING SAID FIRST PREDETERMINED TEMPERATURE; SAID PULVERULENT MATERIAL HAVING SAID FIRST PREDETERMINED TEMPERATURE OF INTRODUCTION MIXING IN SAID CLOSED CHAMBER WITH SAID GASEOUS MEDIUM HAVING SAID SECOND PREDETERMINED TEMPERATURE OF INTRODUCTION AND HAVING SAID SUFFICIENTLY HIGH RELATIVE HUMIDITY SO THAT SAID GASEOUS MEDIUM IS COOLED BY SAID PULVERULENT MATERIAL AND SO THAT MOISTURE CONDENSATION WILL TAKE PLACE ON THE SURFACE OF THE PARTICLES OF SAID PULVERULENT MATERIAL FORMING A THIN FILM OF MOISTURE THEREON THEREBY MAKING SAID PARTICLES SUBSTANTIALLY INCAPABLE OF CARRYING ELECTROSTATIC CHARGES; AND SIFTING SAID PULVERULENT MATERIAL IN SAID CLOSED CHAMBER WHILE THE SAME IS THUS SUBSTANTIALLY FREE OF ELECTROSTATIC CHARGES, WHEREBY A SIFTED MATERIAL IS OBTAINED SUBSTANTIALLY WITHOUT INCREASING THE MOISTURE CONTENT THEREOF. 